Problem compiling Flex project from Flash Builder

I'm having a problem with Adobe Flash Builder, when trying to compile and run a Flex Project I had previously coded outside FB.

I imported the classes following the correct package file system, and everything seemed to work correctly: none of the classes shows errors (and if I code something wrong, an error is shown, so FB error system is working), the auto-complete works fine in all classes, the Main.xmxl class is set up as 'default Application' in the project...

Maybe I skipped a step in between or something, but I always get the following error when trying to compile:

Error
Sun Jun 19 18:40:32 CEST 2011
Uncaught exception in compiler

java.lang.NullPointerException
at flex2.compiler.mxml.lang.StandardDefs.isIUIComponent(StandardDefs.java:784)
at flex2.compiler.mxml.builder.ComponentBuilder.constructComponent(ComponentBuilder.java:177)
at flex2.compiler.mxml.builder.ComponentBuilder.analyze(ComponentBuilder.java:84)
at flex2.compiler.mxml.dom.Node.analyze(Node.java:53)
at flex2.compiler.mxml.builder.VectorBuilder$ElementNodeHandler.componentNode(VectorBuilder.java:173)
at flex2.compiler.mxml.lang.ValueNodeHandler.invoke(ValueNodeHandler.java:73)
at flex2.compiler.mxml.builder.VectorBuilder.processChildren(VectorBuilder.java:142)
at flex2.compiler.mxml.builder.VectorBuilder.analyze(VectorBuilder.java:97)
at flex2.compiler.mxml.dom.VectorNode.analyze(VectorNode.java:50)
at flex2.compiler.mxml.builder.AbstractBuilder$RValueNodeHandler.vectorNode(AbstractBuilder.java:999)
at flex2.compiler.mxml.lang.ValueNodeHandler.invoke(ValueNodeHandler.java:113)
at flex2.compiler.mxml.builder.AbstractBuilder$RValueNodeHandler.process(AbstractBuilder.java:975)
at flex2.compiler.mxml.builder.AbstractBuilder.processRValueNodes(AbstractBuilder.java:941)
at flex2.compiler.mxml.builder.AbstractBuilder.processPropertyNodes(AbstractBuilder.java:770)
at flex2.compiler.mxml.builder.AbstractBuilder.processPropertyNodes(Abstra开发者_JAVA百科ctBuilder.java:741)
at flex2.compiler.mxml.builder.ComponentBuilder$ComponentChildNodeHandler.property(ComponentBuilder.java:506)
at flex2.compiler.mxml.lang.ChildNodeHandler$CoreDeclarationHandler.property(ChildNodeHandler.java:208)
at flex2.compiler.mxml.lang.DeclarationHandler.invoke(DeclarationHandler.java:94)
at flex2.compiler.mxml.lang.ChildNodeHandler$CoreDeclarationHandler.invoke(ChildNodeHandler.java:236)
at flex2.compiler.mxml.builder.ComponentBuilder$ComponentChildNodeHandler.invoke(ComponentBuilder.java:705)
at flex2.compiler.mxml.lang.ChildNodeHandler.scanChildNodes(ChildNodeHandler.java:79)
at flex2.compiler.mxml.builder.ComponentBuilder.processChildren(ComponentBuilder.java:437)
at flex2.compiler.mxml.builder.ComponentBuilder.analyze(ComponentBuilder.java:88)
at flex2.compiler.mxml.dom.Node.analyze(Node.java:53)
at flex2.compiler.mxml.builder.DocumentBuilder$NestedDeclarationNodeHandler.componentNode(DocumentBuilder.java:792)
at flex2.compiler.mxml.lang.ValueNodeHandler.invoke(ValueNodeHandler.java:73)
at flex2.compiler.mxml.builder.DocumentBuilder$DocumentChildNodeHandler.processNestedDeclaration(DocumentBuilder.java:716)
at flex2.compiler.mxml.builder.ComponentBuilder$ComponentChildNodeHandler.nestedDeclaration(ComponentBuilder.java:616)
at flex2.compiler.mxml.builder.DocumentBuilder$DocumentChildNodeHandler.invoke(DocumentBuilder.java:763)
at flex2.compiler.mxml.lang.ChildNodeHandler.scanChildNodes(ChildNodeHandler.java:79)
at flex2.compiler.mxml.builder.ComponentBuilder.processChildren(ComponentBuilder.java:437)
at flex2.compiler.mxml.builder.DocumentBuilder.analyze(DocumentBuilder.java:188)
at flex2.compiler.mxml.dom.Node.analyze(Node.java:53)
at flex2.compiler.mxml.ImplementationCompiler.parse1(ImplementationCompiler.java:183)
at flex2.compiler.mxml.MxmlCompiler.parse1(MxmlCompiler.java:179)
at flex2.compiler.CompilerAPI.parse1(CompilerAPI.java:2872)
at flex2.compiler.CompilerAPI.parse1(CompilerAPI.java:2825)

etc...

Any guesses? I'm really getting mad with it...

Help would be greatly appreciated :)